The sensing of the output current presents nowadays a highly required feature in the DC-to-DC converters design, encountered across all SMPS (switched-mode power supply) device manufacturers. Indeed, the knowledge of the DC-to-DC converter output current is required for the platform development (e.g. optimisation of the software allowing the power consumption reduction in e.g. mobile phone platform). Moreover, information about the output current can also be used to drive the internal functions of the DC-to-DC converter.
In the integrated power supply, we encounter basically two solutions of current sensing. The first relies on external resistance current sensing: the voltage drop on a known resistance (e.g. discrete resistance, parasitic resistance of inductance . . . ) is measured, and the current can be obtained by using ohms law. However, this technique suffers from several drawbacks. Firstly, the extra resistance introduced in the power path considerably decreases the power efficiency of the DC-to-DC converter. Secondly, when a parasitic resistance is used for the current sensing, a complicated calibration mechanism is to be implemented, notably for compensating the variation of the resistance, due to the temperature variation. In practice, it is complicated (say impossible) to compensate perfectly. If the calibration system is not implemented, a circuit using parasitic resistance sensing provides only relative information about output current.
The second technique is based on the N or P-MOS drain-to-source voltage sensing and its recopy onto known MOS resistance (copy-MOS transistor). This technique is useful for obtaining an absolute value of output current at a given time, but it does not allow obtaining the average value of the output current in a simple manner, which is generally required. Furthermore, this technique suffers from low accuracy, namely at the high switching frequency of the DC/DC converter. This is due to the limitation of high-speed operation amplifier inside the current-replica circuit.
Thus, the above mentioned techniques suffer generally from one of the following drawbacks: lack of the absolute value information, high complexity of the circuit for measuring the current, low accuracy or decrease of the power efficiency of the DC-to-DC converter.